Many cranes and other hoists that handle critical loads, such as for steel mill use, require failure-proofing safety measures because the potential consequences of dropping a load may be serious. U.S. Pat. Nos. 4,175,727 and 4,177,973 disclose safety systems which enable a brake to engage the drum in a hazard condition, such as when there is a discontinuity between the rotational inputs from two inputs to an out-of-sync detector device.
U.S. Pat. No. 3,836,121 shows a hoist with a safety system in which an independently drive worm is continuously meshed with a worm wheel on the drum. The drive to the worm is insufficient to power the worm wheel during normal operation. In the event of a failure in the drive train to the drum, the worm automatically locks up the worm wheel to prevent dropping of the load.